Quantitative Chemically Specific Coherent Diffractive Imaging of Reactions at Buried Interfaces with Few Nanometer Precision

Abstract

Characterizing buried layers and interfaces is critical for a host of applications in nanoscience and nano-manufacturing. Here we demonstrate non-invasive, non-destructive imaging of buried interfaces using a tabletop, extreme ultraviolet (EUV), coherent diffractive imaging (CDI) nanoscope. Copper nanostructures inlaid in SiO2 are coated with 100 nm of aluminum, which is opaque to visible light and thick enough that neither optical microscopy nor atomic force microscopy can image the buried interfaces. Short wavelength (29 nm) high harmonic light can penetrate the aluminum layer, yielding high-contrast images of the buried structures. Moreover, differences in the absolute reflectivity of the interfaces before and after coating reveal the formation of interstitial diffusion and oxidation layers at the Al-Cu and Al-SiO2 boundaries. Finally, we show that EUV CDI provides a unique capability for quantitative, chemically-specific imaging of buried structures, and the material evolution that occurs at these buried interfaces, compared with all other approaches.

Observation of ionization enhancement in two-color circularly polarized laser fields

“…In the simplest semi-classical picture of HHG, the electron can return to the parent ion with high kinetic energy and then any excess energy greater than the ionization potential can then be emitted as a high-harmonic photon. When the HHG process is properly phase matched, a bright coherent beam of extreme ultraviolet (EUV) or soft X-ray light is generated [6][7][8][9][10], which can be used to uncover coupled dynamics in materials with femtosecond-to-attosecond temporal resolution [11][12][13][14][15], and can also be used for high-resolution imaging [16][17][18][19]. Alternatively, if the electron does not recombine upon re-encountering the ion it may rescatter from the ion, encoding information about the sub-ångström and sub-femtosecond structure of the scattering potential into the photoelectron momentum distribution [20][21][22].…”

Phase matching of noncollinear sum and difference frequency high harmonic generation above and below the critical ionization level

“…We assume a transmission of 1% for the spectrometer. This gives a harmonic flux of ~10 6 photons/sec/harmonic emitted from the source. We compare our estimated high-energy circularly polarized harmonic flux to that obtained in the collinear geometry by Fan, et al [26].…”

“…4) are generated with usable flux (~10 6 photons/sec/harmonic above the aluminum edge at ~70 eV, see Appendix G). We note that this flux is an order of magnitude lower than the flux that has been attained in high energy collinear circularly polarized HHG (~5×10 7 photons/sec/harmonic [26]).…”

“…HHG is an ideal source for many applications, including nanoscale imaging [6], studies of nanoscale transport [7], ultrafast photoelectron spectroscopy [8][9][10][11][12], and element specific characterizations of ultrafast dynamics [13][14][15][16][17].…”

Transverse Coherence Limited Coherent Diffraction Imaging using a Molybdenum Soft X-ray Laser Pumped at Moderate Pump Energies

“…Table-top XUV sources employing high-order harmonic generation (HHG) 4 have proven their relevance to the field by allowing a broader access to short wavelength imaging and holography 5 and the implementation of more complex experimental schemes. The reflection geometry 6 for instance has become a technique for surface structure analysis 7, 8 and biologic specimen classification 9 and promises many new applications in the near future. A major drawback of HHG sources often addressed is the limited flux demanding for integration time of several seconds up to hundreds of seconds 10–15 depending on the anticipated resolution of the XUV microscope.…”